Since 1996 Nichia Corporation in Japan had developed a blue-LED to excite a phosphor powder to emit yellow light, then the blue light and the yellow light are mixed to produce a white light with high intensity. This development begins the era of white-LED application.
A white light is composed of different color lights. At least two color lights with different wavelength are required to be mixed so as to make human eyes to feel as a white light. For example, red, blue and green lights, or blue and yellow lights can be mixed to make human eyes feel as a white light. According to this principle, a white-LED can be designed.
Conventional white light of three wavelengths requires at least three different phosphor powders in order to have high color rendering index. Three different phosphor powders must have the same ability to absorb the exciting light, the absorbing coefficients of the three phosphor powders have to be nearly the same, and the quantum efficiency of energy conversion of the three different phosphor powders must be as close as possible, the proportion between the three different phosphor powders must be adjusted carefully to get an ideal white light. However, it is difficult to choose three kinds of phosphor powders described as above.
Furthermore, the specific gravity, the granule size and the luminescent efficiency of the three different phosphor powders are not the same, and thus uniformly mixing of three different phosphor powders is not easy. Mass production requires classified inspection for correlated color temperature, color rendering index and chromaticity coordinate, therefore it causes low yield. In addition, multiple phosphor powders have the problem of mutual interference so as to lower the luminescent efficiency thereof as compared with mono phosphor powder. Up to the present, it is seldom for conventional white-LED to use multiple phosphor powders to cooperate with an LED chip to emit white light.
In a mono phosphor powder system, the main design is using a blue-LED to cooperate with a yellow phosphor powder (YAG or TAG) to generate a white light. The disadvantages thereof are the higher correlated color temperature that causes uncomfortableness for human eyes and weaker red light that causes low color rendering index (70˜80) and low color saturation.
Recently, LED goes forward to the application of backlight source for LCD monitor and lighting. The International Commission on Illumination (CIE) sets up a standard that a light source with color rendering index 80˜90 is suitable for indoor lighting. Therefore a blue chip cooperated with a yellow phosphor powder (YAG or TAG) is not suitable for indoor lighting.
Moreover, the backlight source used for LCD monitor requires high color saturation. The ideal proportion of luminescent energy between red light (650 nm), green light (525 nm) and blue light (450 nm) is 1:1:1 so as to generate a variety of colors for real perfect appearance of pictures. However, a blue-LED to cooperate with a yellow phosphor powder just achieves a proportion of luminescent energy between red light, green light and blue light by 0.4:0.6:1.0, which is not suitable as a good backlight source for LCD monitor use.
Currently, a white LED of backlight source for LCD monitor uses three chips of red, green, blue, but more chips causes more costs. The red chip decays faster than blue chip and green chip. When the temperature of the backlight source increases, the red light decays 23%, while green light and blue light decays 7%, thus the color saturation becomes poor, and then lose real perfect appearance of pictures.